1. Field of the Invention
This invention relates to a continuously variable transmission for use in various vehicles such as automobiles or various industrial machines, and particularly to the yoke thereof and improvements in the engagement between the yoke and a support post or a trunnion.
2. Related Background Art
Continuously variable transmissions of various types have been used and one of them is a toroidal type continuously variable transmission. This is a transmission in which the opposed surfaces of an input disc mounted on an input shaft and an output disc mounted on an output shaft are formed by toroidal surfaces. A power roller is disposed between these toroidal surfaces, and by the oscillated state (angle) thereof being changed, the transmission ratio between the input shaft and the output shaft can be changed.
FIGS. 1 and 2 of the accompanying drawings show a toroidal type continuously variable transmission of the single cavity type described in Japanese Patent Publication No. 8-19998. It includes an input shaft 15, an input disc 2 and an output disc 4 disposed thereon, yokes 20a and 20b supported by a casing 22, trunnions 6 supported by the casing 22 and the yokes 20a, 20b, a power roller 8 supported by the trunnions 6 and located between the two discs, etc.
The input side disc 2 and the output side disc 4 are rotatably supported around the tubular input shaft 15 through needle bearings 16. Also, a cam plate 10 is spline-engaged with the outer peripheral surface of an end portion (the left end portion as viewed in FIG. 1) of the input shaft 15, and is prevented from moving away from the input side disc 2 by a flange portion 17. This cam plate 10 and rollers 12 together constitute a pressing device 9 of the loading cam type for rotating the input side disc 2 while pressing the input side disc 2 toward the output side disc 4 on the basis of the rotation of the input shaft 15. An output gear 18 is coupled to the output side disc 4 by keys 19 so that the two may be rotated in synchronism with each other.
Pivot shafts 5 provided on the upper and lower end portions of the pair of trunnions 6 extending vertically as viewed in FIG. 2 are supported for oscillation relative to the pair of yokes 20a and 20b extending horizontally and for displacement in the axial direction (the front-to-back direction as viewed in FIG. 1, or the vertical direction as viewed in FIG. 2). That is, the pivot shafts 5 are rotatably supported inside circular holes 25 formed in the left and right end portions of the yokes 20a and 20b, through radial needle bearings 26. The outer peripheral surfaces of the outer races 27 of these radial needle bearings 26 are made into spherical convex surfaces and are fitted inside the circular holes 25 for oscillation and axial displacement.
The yokes 20a and 20b are provided parallel to each other between the input side disc 2 and the output side disc 4 so as to sandwich the input shaft 15 therebetween. Such yokes 20a and 20b are formed into an elliptical shape or a long rectangular shape as shown in FIG. 4 of the accompanying drawings by subjecting a thick steel plate to the forging work of punching it, and have sufficient rigidity.
These yokes 20a and 20b are supported for some displacement on the inner surface of the casing 22 containing the body of the continuously variable transmission therein. That is, restraining holes 21a and 21b are formed in the intermediate portions of the yokes 20a and 20b, respectively, and a pair of support posts 23a and 23b are secured on the same straight line at the vertically opposite sides of the inner surface of the casing 22. The outer peripheral surface of the tip end portion of one (lower as viewed in FIG. 2) support post 23a is made into a spherical convex surface, and the inner peripheral surface of the restraining hole 21a formed in the intermediate portion of the yoke 20a which corresponds thereto is made into a spherical concave surface, and the tip end portion of the support post 23a is fitted in this restraining hole 21a. Accordingly, the yoke 20a is supported on the tip end portion of the support post 23a for oscillation and displacement.
In contrast, the outer peripheral surface of the intermediate portion of the other (upper as viewed in FIG. 2) support post 23b is made into a rectangular cylindrical shape, and the inner peripheral surface of the restraining hole 21b formed in the intermediate portion of the other yoke 20b which corresponds thereto is made into a rectangular cylindrical shape in which the longer side of the cross-sectional shape is longer than the longer side of the cross-sectional shape of the intermediate portion of the support post 23b. Accordingly, a gap 24 as shown in FIG. 4 is present between the inner peripheral surface of the restraining hole 21b and the outer peripheral surface of the intermediate portion of the yoke 20b in a state in which the yoke 20b is fitted on the intermediate portion of the support post 23b. As the result, the yoke 20b is supported for oscillatory displacement about a line X--X and some displacement in a lengthwise direction relative to the support post 23b.
Circular holes 28 are formed in the intermediate portions of the trunnions 6 supported for oscillation and axial displacement inside the casing 22 through the yokes 20a and 20b in the manner described above, and displaceable shafts 7 are supported therein. These displaceable shafts 7 have support shaft portions 29 parallel to each other and eccentric with respect to each other and pivot shaft portions 30, and are provided at opposite sides of 180.degree. with the input shaft 15 therebetween. The support shaft portions 29 are rotatably supported inside the circular holes 28 through radial needle bearings 31, and the power rollers 8 are rotatably supported around the pivot shaft portions 30 through radial needle bearings 32.
The direction in which the pivot shaft portions 30 of the displaceable shafts 7 are eccentric relative to the support shaft portions 29 is the same direction with respect to the direction of rotation of the input side and output side discs 2 and 4 (the inverse vertical direction as viewed in FIG. 2) and is the direction substantially orthogonal to the direction of disposition of the input shaft 15 (the horizontal direction as viewed in FIG. 1 or the front to back direction as viewed in FIG. 2). Accordingly, the power rollers 8 are supported for some displacement in the direction of disposition of the input shaft 15. As the result, even when the power rollers 8 tend to be displaced axially of the input shaft 15 due to the fluctuation or the like of the amount of elastic deformation of each constituent member based on the fluctuation of torque transmitted by the toroidal type continuously variable transmission, this displacement can be absorbed without any unreasonable force being applied to each constituent part.
Also, thrust ball bearings 33 and thrust bearings 34 are provided between the outer sides of the power rollers 8 and the inner sides of the intermediate portions of the trunnions 6. The thrust ball bearings 33 support a load in the thrust direction applied to the power rollers 8 and yet permit the rotation thereof. Also, the thrust bearings 34 support a thrust load applied to the outer races 35 of the thrust ball bearings 33 and yet permit the pivot shaft portions 30 and the outer races 35 to oscillate about the support shaft portions 29.
Further, driving rods 36 are coupled to one end portion (the lower end portion as viewed in FIG. 2) of the trunnions 6, and driving pistons 37 secured to the outer peripheral surfaces of the intermediate portions thereof are oil-tightly fitted in driving cylinders 38. These driving pistons 37 and driving cylinders 38 together constitute an actuator for displacing the trunnions 6 axially of the pivot shafts 5. Also, oil pressure can be supplied into and discharged from the driving cylinders 38 on the basis of the changeover of a control value 39.
During the operation of the toroidal type continuously variable transmission, the rotation of the input shaft 15 is transmitted to the input side disc 2 through the pressing device 9, is further transmitted to the output side disc 4 through the pair of power rollers 8, and is taken out from the output gear 18.
When the rotational speed ratio between the input shaft 15 and the output gear 18 is to be changed, the pair of driving pistons 37 are displaced by the same distance in opposite directions on the basis of the changeover of the control value 39, whereupon the pair of trunnions 6 are displaced in opposite directions (the right power roller 8 in FIG. 2 is downwardly displaced and the left power roller 8 is upwardly displaced). As the result, the direction of a force in the tangential direction acting on the portion of contact between the peripheral surfaces 8a of the power rollers 8 and the inner sides 2a and 4a of the input side disc 2 and the output side disc 4 changes and along therewith, the trunnions 6 oscillate in opposite directions about the pivot shafts 5 pivotally supported by the yokes 20a and 20b. As the result, the position of contact between the peripheral surfaces 8a of the power rollers 8 and the inner sides 2a and 4a of the discs 2 and 4 changes and the rotational speed ratio between the input shaft 15 and the output gear 18 changes.
At this time, the yoke 20b is displaced within the range of the gap 24 relative to the support post 23b, whereby the displacement of the driving pistons 37 in the driving cylinders 38 may be effected smoothly. However, there occurs a case where the contact pressure (pressing force) of the portion of contact between the peripheral surfaces 8a of the power rollers 8 supported on the inner sides of the trunnions 6 and the inner sides 2a and 4a of the input side and output side discs 2 and 4 differs between the diametrally opposite sides of the power rollers 8 (the input disc side and the output disc side). This difference in the contact pressure leads to the difference between the operation traction coefficients of the traction portions, and further to the difference between the sliding rates of these traction portions. As the result, the stability of transmission synchronism regarding the pair of power rollers 8 may be spoiled or a greater force may be applied to one traction portion of the input disc side or the output disc side than to the other traction portion, whereby the durability thereof may be spoiled.
The known toroidal type continuously variable transmission also include a so-called double cavity type one in which two sets of input side discs, output side discs and power rollers are provided and these are disposed in parallel to each other relative to the direction of transmission of power. FIGS. 5 to 7 of the accompanying drawings shown an example of it (an example described in Japanese Patent Publication No. 8-23386).
An input shaft 101a is supported for rotation only inside a casing 105, and a tubular transmission shaft 138 is supported around and concentrically with this input shaft 101a for rotation relative thereto. First and second input side discs 116 and 117 are supported on both sides of the intermediate portion of the transmission shaft 138 through ball splines 118 with their inner sides 102a opposed to each other. Accordingly, the first and second input side discs 116 and 117 are disposed concentrically with each other and are rotatable in synchronism with each other.
Also, first and second output side discs 119 and 120 are supported around the intermediate portion of the transmission shaft 138 through a sleeve 121. This sleeve 121 has an output gear 122 integrally provided on the outer peripheral surface of the intermediate portion thereof, and has an inner diameter larger than the outer diameter of the transmission shaft 138, and is supported on a support wall 123 concentrically with the transmission shaft 138 and for rotation only by a pair of antification bearings 124. The first and second output side discs 119 and 120 are spline-engaged with the opposite end portions of the sleeve 121 with their inner sides 4a opposed to each other. Accordingly, the first and second output side discs 119 and 120 are supported concentrically with the first and second input side discs 116 and 117 and for rotation independently thereof with their inner sides 104a opposed to the inner side 102a of one of the first and second input side discs 116 and 117.
A pair of yokes 125a and 125b are supported on the inner surface of the casing 105 and sideways of the first and second input side discs 116 and 117 so as to sandwich them. These yokes 125a and 125b are formed into a rectangular frame-like shape as shown in FIG. 8 by subjecting a metal plate such a steel plate to pressing work or forging work. Circular support holes 126 for pivotally supporting first and second pivot shafts 133 and 134 provided on the opposite end portions of first and second trunnions 131 and 132 which will be described later are formed in the four corners of each of the yokes 125a and 125b, and circular restraining holes 127 are formed in the widthwise (horizontal as viewed in FIG. 6 or vertical as viewed in FIG. 8) central portion of the axial (horizontal as viewed in FIGS. 5 and 8) opposite end portions of the transmission shaft 138 so as to be located between adjacent ones of the support holes 126. The pair of yokes 125a and 125b are supported by support posts 128a and 128b formed on the inner surface of the casing 105. These support posts 128a and 128b are provided in opposed relationship with a first cavity 129 between the inner side 102a of the first input side disc 116 and the inner side 104a of the first output side disc 119 and a second cavity 130 between the inner side 102a of the second input side disc 117 and the inner side 104a of the second output side disc 120, respectively.
Accordingly, the yokes 125a and 125b are supported by the support posts 128a and 128b and have their one end portion opposed to the outer peripheral portion of the first cavity 129 and have their other end portion opposed to the outer peripheral portion of the second cavity 130.
Also, a pair of first trunnions 131 are disposed in the first cavity 129 at the diametrally opposite positions of the first input side disc 116 and the first output side disc 119, and a pair of second trunnions 132 are disposed in the second cavity 130 at the diametrally opposite positions of the second input side disc 117 and the second output side disc 120. First pivot shafts 133 (four in total) provided concentrically with one another on the opposite end portions of the first trunnions 131 are supported for oscillation and axial displacement on the widthwise opposite sides of one end portion of the pair of yokes 125a and 125b, as shown in FIG. 6. That is, inside the support holes 126 formed in one end portion of the yokes 125a and 125b, the first pivot shafts 133 are supported by radial needle bearings 135 comprising an outer race 136 of which the outer peripheral surface is a spherical convex surface and the inner peripheral surface is a cylindrical surface, and a plurality of needles 137. Also, the second trunnions 132 are supported in the second cavity 130 by a structure similar to the first trunnions 131.
As shown in FIG. 6, circular holes 139 are formed in the intermediate portions of the first and second trunnions 131 and 132, and first and second displaceable shafts 140 and 141 are supported therein. The first and second displaceable shafts 140 and 141 have support shaft portions 142 and pivot shaft portions 143 parallel and eccentric to each other, and the support shaft portions 142 are rotatably supported inside the circular holes 139 through radial needle bearings 144. First and second power rollers 145 and 146 are rotatably supported around the pivot shaft portions 143 through radial needle bearings 147.
The pair of first displaceable shafts 140 and the pair of second displaceable shafts 141 provided in the first and second cavities 129 and 130, respectively, are opposed to the input shaft 101a and the transmission shaft 138 on the opposite sides of 180.degree.. Also, the directions in which the pivot shaft portions 143 of the first and second displaceable shafts 140 and 141 are eccentric relative to the support shaft portions 142 are the same directions (vertically opposite directions as viewed in FIG. 6) with respect to the direction of rotation of the first and second input side discs 116 and 117 and the first and second output side discs 119 and 120, and are directions substantially orthogonal to the direction of disposition of the input shaft 115. Accordingly, the first and second power rollers 145 and 146 are supported for some displacement in the direction of disposition of the input shaft 101a and the transmission shaft 138.
As the result, even when due to the fluctuation or the like of the amount of elastic deformation of each constituent member based on the fluctuation of torque transmitted by the toroidal type continuously variable transmission, the first and second power rollers 145 and 146 tend to be displaced in the axial direction (horizontal direction as viewed in FIG. 5 or the front-to-back direction as viewed in FIG. 6) of the input shaft 101a and the transmission shaft 138, this displacement can be absorbed without any unreasonable force being applied to each constituent member.
Also, thrust ball bearings 148 and thrust bearings 149 such as slide bearings or needle bearings are provided between the outer sides of the first and second power rollers 145 and 146 and the inner sides of the intermediate portions of the first and second trunnions 131 and 132. The thrust ball bearings 148 support loads in the thrust direction applied to the first and second power rollers 145 and 146, and yet permit the rotation thereof. Also, the thrust bearings 149 support thrust loads applied from the first and second power rollers 145 and 146 to the outer races 150 of the thrust ball bearings 148, and yet permit the pivot shaft portions 143 and the outer races 150 to oscillate about the support shaft portions 142.
Further, driving rods 151 are coupled to one end portion (the lower end portion as viewed in FIG. 6) of the first and second trunnions 131 and 132, and driving pistons 152 are secured to the outer peripheral surfaces of the intermediate portions thereof. These driving pistons 152 are oil-tightly fitted in driving cylinders 153. These driving pistons 152 and driving cylinders 153 together constitute an actuator for displacing the first and second trunnions 131 and 132 axially of the first and second pivot shafts 133 and 134. Also, pressurized oil can be supplied into and discharged from the driving cylinders 153 on the basis of the changeover of a control value (not shown).
Further, a pressing device 110 of the loading cam type is provided between the input shaft 101a and the first input side disc 116. This pressing device 110 is comprised of a cam plate 111 spline-engaged with the intermediate portion of the input shaft 101a and also supported against axial displacement and rotatable with the input shaft 101a, and a roller 113. The pressing device 110 presses the first input side disc 116 toward the second input side disc 117 on the basis of the rotation of the input shaft 101a and yet rotates it.
During the operation of the above-described double cavity toroidal type continuously variable transmission, the rotation of the input shaft 101a is transmitted to the first input side disc 116 through the pressing device 110, and the second input side disc 117 is rotated in synchronism therewith. The rotation of these first and second input side discs 116 and 117 is transmitted to the first and second output side discs 119 and 120 through the first and second power rollers 145 and 146 in the first and second cavities 129 and 130, and the rotation thereof is taken out from the output gear 122.
When the rotational speed ratio between the input shaft 101a and the output gear 122 is to be changed, the pairs of driving pistons 152 provided correspondingly to the first and second cavities 129 and 130 are displaced by the same distance in opposite directions in each of the cavities 129 and 130 on the basis of the changeover of the control value.
Along with the displacement of these driving pistons 152, two pairs of (four in total) trunnions 133 and 134 are displaced in opposite directions (the right first and second power rollers 145 and 146 as viewed in FIG. 6 downwardly and the left first and second power rollers 145 and 146 upwardly). As the result, the direction of a tangential force acting on the portions of contact between the peripheral surfaces 109a of the first and second power rollers 145 and 146 and the inner sides 102a and 104a of the first and second input side discs 116 and 117 and the first and second output side discs 119 and 120 changes and along therewith, the first and second trunnions 131 and 132 oscillate in opposite directions about the first and second pivot shafts 133 and 134 pivotally supported on the yokes 125a and 125b. As the result, the positions of contact between the peripheral surfaces 109a and 109b of the first and second power rollers 145 and 146 and the inner sides 102a and 104a of the discs 116, 117, 119 and 120 change, and the rotational speed ratio between the input shaft 101a and the output gear 122 changes.
In the case of the prior-art structure shown in FIGS. 5 to 8, however, the work of supporting the pair of yokes 125a and 125b inside the casing 105 and supporting the first and second trunnions 131 and 132 on these yokes has been cumbersome, and this has been the cause of the increased manufacturing cost of the double cavity toroidal type continuously variable transmission.
Firstly, the yokes 125a and 125b are supported relative to the casing 105 by the two support posts 128a and 128b, respectively. Therefore, not only the manufacturing cost of these support posts 128a and 128b is increased, but also the work of mounting these support posts 128a and 128b with respect to the casing 105 and the work of supporting the yokes 125a and 125b relative to these support posts 128a and 128b has become cumbersome.
Secondly, the support holes 126 formed at the four corners of the yokes 125a and 125b are mere circular holes, and the outer races 136 of the radial needle bearings 135 supporting the first and second trunnions 131 and 132 have been fitted in the support holes 126 without any backlash. Therefore, the work of fitting the outer races 136 into the support holes 126 and supporting the first and second trunnions 131 and 132 relative to the yokes 125a and 125b is cumbersome, and it is difficult to enhance the efficiency of the assembling work for the continuously variable transmission.
Thirdly, during the operation of the continuously variable transmission, the inner peripheral surfaces of the support holes 126 and the outer peripheral surfaces of the outer races 136 may slide over the entire periphery and therefore, it is necessary to finish (polish) the inner peripheral surfaces of the support holes 126 over the entire periphery, and the work is cumbersome and the cost increases.
The structure of the double cavity toroidal type continuously variable transmission in which four trunnions are supported by a pair of yokes is described not only in the aforementioned Japanese Patent Publication No. 8-23386, but also in Japanese Laid-Open Patent Application No. 2-283949, Japanese Laid-Open Patent Application No. 5-126222 and Japanese Laid-Open Patent Application No. 6-34010. However, any of these suffers from a similar problem, or the support by the yokes is unstable and during the transition of speed change, the state of contact between the inner sides 102a and 104a of the discs and the peripheral surfaces 109a of the power rollers may become unstable.